Paper-Making Aid Composition and Process for Increasing Tensile Strength of Paper

ABSTRACT

A paper-making aid composition, as well as processes for increasing tensile strength in paper and paper-making processes are provided. The paper-making composition comprises an anionic dialdehyde-modified polyacrylamide and a polyamide polyamine-epichlorohydrin (“PAE”) resin present in the paper-making aid composition at a weight ratio of PAE resin to anionic dialdehyde-modified polyacrylamide of from about 5:1 to about 1:1.6. The processes utilize an anionic dialdehyde-modified polyacrylamide and PAE resin at certain ratios. The anionic dialdehyde-modified polyacrylamide and PAE resin may be added to the pulp slurry as a composition or separately.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Chinese Patent Application SerialNo. 201410698600.X filed on Nov. 27, 2014, the disclosure of which isincorporated herein by reference in its entirety.

TECHNICAL FIELD OF THE INVENTION

The present invention, which is in the field of paper-making process,relates to a paper-making aid composition and its preparation process,to a process for increasing tensile strength, especially dry tensilestrength and wet tensile strength of paper, and to a paper-makingprocess.

BACKGROUND OF THE INVENTION

Strength (including dry strength, wet strength and temporary wetstrength, etc.) is a structural property of paper, which mainly dependson the interfiber bond and the nature of fibers in the paper sheet.During the paper making process, the strength properties of paper can beimproved by adding strength agents to the paper stock by adjusting theratio of fibers and pulping to fibrillate as well as by virtue offilm-forming properties of a surface sizing agent. Strength agents havebecome increasingly important as they can improve the strengthproperties of paper and avoid the defect of pulping which accompaniedwith a deterioration in drainage property of paper stock and change thepaper properties. Furthermore, the paper and paper board will losealmost all mechanical strength after soaking in water and only 4%-10% ofdry paper strength can be retained due to the interfiber bonding forcefrom hydrogen bonding in celluloses. However, some types of paper arerequired to not only possess a certain dry strength but also retainnotable mechanical strength and characteristics after being soaked inwater, such as photo paper, military map paper, banknote paper, papertableware, wallpaper base, etc. In this case, a special strength agentcan be added to the paper to impart it with essential wet strength.Therefore, a strength agent is usually divided into a dry strength and awet strength agent.

At present, popular dry strength agents include natural polymers, suchas cationic starch, Carboxymethyl Cellulose (CMC) and guar gum, andsynthetic polymers such as polyacrylamide (cationic, anionic andamphoteric), glyoxalated polyacrylamides (GPAMs), polyvinylamine, etc.Di-aldehyde functionalized polyacrylamide, which was prepared fromdi-aldehyde and polyacrylamide, was developed first as a temporary wetstrength resin (see U.S. Pat. No. 3,556,932A, U.S. Pat. No. 4,605,702A)back in the 1970's and 1980's. It was then developed as a dry strengthresin used in combination with a wet strength resin. In this category ofdi-aldehyde functionalized polyacrylamide, glyoxalated polyacrylamide(GPAM), prepared from glyoxal and backbone polyacrylamide, is the mostcommonly used dry strength aid. Anionic and amphoteric (WO0011046A1), aswell as cationic (U.S. Pat. No. 7,641,766B2, U.S. Pat. No. 7,901,543B2)di-aldehyde functionalized polyacrylamides (mostly GPAMs) usually couldbe used separately, and they were developed to impart paper withenhanced dry strength, wet strength or drainage ability.

Di-aldehyde functionalized polymers, including cationic, anionic andamphoteric acrylamide polymers, particularly the glyoxalated modifieddiallyldimethylammonium chloride (DADMAC)/acrylamide copolymers, arewidely used as a dry strength and temporary wet strength aid in thepaper and paperboard area. Such polymer strength aid is of greatinterest for paper and paperboard manufacturer since (1) it providesgood temporary wet strength together with good dry strength, and (2) italso helps improve the drainage ability and the paper machinerunnability.

Wet strength agents are commonly used in the paper-making industry,including polyamide polyamine-epichlorohydrin (PAE) resin,melamine-formaldehyde (MF) resin, urea-formaldehyde (UF) resin and othertypes of wet strength agents. MF resin and UF resin can't be used widelysince each can only be used in acidic conditions and contains harmfuland volatile formaldehyde. Another type of wet strength agent, such aspolyethyleneimine (PEI), has not been commercialized in a large scaledue to the unrefined technology. PAE resin is a water-soluble, cationicand thermosetting resin featured with good wet strength property,formaldehyde-free, less yellowing of paper, easy to use, and the like,so it is particularly suitable for hand sheet in neutral and alkalinecondition. However, if the PAE resin dosage exceeds a certain range, theincrease in wet strength of paper will be greatly reduced. To some typesof paper which requires a relatively high wet strength, the one-sidedexcessive addition of PAE resin cannot achieve the effect of impartinghigh wet strength. Moreover, PAE resin has drawbacks including a longtime for curing, difficult for waste paper recovery, high level oforganochlorine, and not environmentally friendly.

Thus, there exists a need to develop a new solution and technology,which has both dry and wet strength performance. Research has beenconducted in this area and has brought forward a series of technicalsolutions. For example, U.S. Pat. No. 5,427,652 disclosed that bycombining a cationic dialdehyde functionalized polyacrylamide (GPAM) andPAE resin, the paper is equipped with the wet tensile strength as wellas the good decomposition performance in the paper recycling process.

U.S. Pat. No. 6,294,645B1 reported a dry strength agent for paper whichcomprises PAE, amphoteric polyacrylamide and wet strength resin, whereinthe wet strength resin could be GPAM. Furthermore, U.S. Pat. No.5,783,041 also disclosed a dry strength agent for paper, comprising PAEresin, glyoxylated cationic polyacrylamide copolymer, and a high chargedensity cationic polymer resin.

WO0011046 disclosed the synthesis of amphoteric and anionic glyoxylatedpolyacrylamide copolymers, and suggested that the anionic or amphotericGPAM could be used alone or in combination with a cationic promoter,wherein the cationic promoter could be starch, PAE resin, polyamines.However, this document does not pay special attention to the optimizedcombination of anionic GPAM with PAE resin.

In addition, CN103215853 disclosed a wet strength agent, and pointed outthat the combination of a permanent wet strength agent with a temporarywet strength agent in a ratio between 0.02˜0.5:0.2˜5 could impart to thepaper both wet tensile strength and good water solubility, wherein thepermanent wet strength agent could be PAE resin and the temporary wetstrength agent could be glyoxylated polyacrylamide resin.

However, there remains a need to optimize the strength agent in theprior art, especially in terms of composition and amount, so as tofurther improve the utilization of the paper strength agent, reduce costand decrease the negative impacts on the environment caused by usingpermanent resins such as PAE resin in large quantities.

SUMMARY OF THE INVENTION

The inventors have performed intensive research and surprisingly foundthat the combination of a polyamide polyamine-epichlorohydrin (PAE)resin with an anionic dialdehyde-modified polyacrylamide (GPAM) strengthagent in a specific PAE resin to GPAM strength-agent mass ratio of about5:1 and about 1:1.6 can significantly increase dry strength and wetstrength of paper, while maintaining other advantageous properties ofthe paper.

It may be advantageous when each of the anionic dialdehyde-modifiedpolyacrylamide and the polyamide polyamine-epichlorohydrin, i.e. theactive substances, is contained in the paper-making aid composition inan amount ranging from 1 to 50 mass-%, preferably 10 to 30 mass-%.

Furthermore, the present inventors have unexpectedly found that, thedry/wet strength of paper could be further improved by separate feedingof the anionic dialdehyde-modified polyacrylamide (GPAM) strength agentand the polyamide polyamine-epichlorohydrin (PAE) resin as compared withthe pre-mixing addition method.

One aspect of the present invention is to provide a paper-making aidcomposition, comprising an anionic dialdehyde-modified polyacrylamide(GPAM) and a polyamide polyamine-epichlorohydrin (PAE) resin, whereinthe mass ratio of the PAE resin to the anionic GPAM is between about 5:1and about 1:1.6. Preferably, the composition does not contain anamphoteric or cationic dialdehyde-modified polyacrylamide. Morepreferably, the composition may only consist of these two components andwater as a medium.

Further, the mass ratio of the PAE resin and the anionicdialdehyde-modified polyacrylamide is between about 3.5:1 and about1:1.6, preferably between about 2:1 and about 1:1.23, more preferablybetween about 1.2:1 and about 1:1.

Aspects of the present invention provide a process to increase strengthof paper, especially dry strength and wet strength, wherein thementioned paper-making aid composition is added to the liquor comprisingpulp in the paper-making process. In one preferred embodiment, thecomponents of above strength aid composition, especially the polyamidepolyamine-epichlorohydrin (PAE) and the anionic dialdehyde-modifiedpolyacrylamide (GPAM), are dosed into the pulp in the separate additionmanner (i.e., they are not pre-mixed or not added simultaneously). Inthe context of this application, “in the separate addition manner”,which is different from pre-mixing addition method or simultaneousaddition method, means that the components are added in sequence with acertain interval, and particularly means that the two main componentsPAE and GPAM are added separately.

Another aspect of the present invention is to provide a paper-makingprocess, comprising the steps of:

-   -   (a) providing a pulp slurry; simultaneously or before or after    -   (b) providing the above paper-making aid composition;    -   (c) adding the paper-making aid composition into the paper        slurry to obtain a paper stock;    -   (d) forming the paper stock obtained in the step (c) to obtain a        wet paper web;    -   (e) pressing and draining the wet paper web obtained in the        step (d) to obtain a wet paper sheet; and    -   (f) drying the wet paper sheet obtained in the step (e) to        obtain a paper sheet.

It would be appreciated that in the above process for increasing thetensile strength of paper and the paper-making process, there is nospecial limitation to the addition manner of PAE resin and anionic GPAM.The two components can be added to a pulp separately or simultaneously,or the two components can be first mixed with each other to form apre-mixed strength agent and then added into a pulp. However, it ispreferred that the two components are fed in the separate additionmethod as described above.

1. Anionic Dialdehyde-Modified Polyacrylamide

In the context, the dialdehyde-modified polyacrylamide is a strengthagent for paper-making, which may be prepared by modifying a basepolymer of polyacrylamide type with a dialdehyde. The dialdehydemodified polyacrylamide-type strength agents are usually used as drystrength agents while some of them can be used to provide the paper withwet strength and drainage properties.

The dialdehyde-modified polyacrylamide used herein is anionic.Correspondingly, the polyacrylamide-type base polymer is also anionic.

The anionic polyacrylamide-type base polymer is a copolymer of one ormore acrylamide monomer(s) and one or more anionic monomer(s). Forexample, the anionic polyacrylamide-type base polymers disclosed inWO0011046A1 are applicable to the present invention and correspondingdialdehyde-modified polyacrylamides, and their preparation methods. Thecontents disclosed in this document are all incorporated herein byreference.

“Acrylamide monomer” means the monomer of formula

wherein R₁ is H or C₁-C₄ alkyl and R₂ is H, C₁-C₄ alkyl, aryl orarylalkyl. Preferably, acrylamide monomers are, for example, acrylamideor methacrylamide.

The following definitions apply in the context:

“Alkyl” means a monovalent group derived from a straight or branchedchain saturated hydrocarbon by the removal of a single hydrogen atom.Representative alkyl groups include methyl, ethyl, n- and iso-propyl,cetyl, and the like.

“Alkylene” means a divalent group derived from a straight or branchedchain saturated hydrocarbon by the removal of two hydrogen atoms.Representative alkylene groups include methylene, ethylene, propylene,and the like.

“Aryl” means an aromatic monocyclic or multicyclic ring system of about6 to about 10 carbon atoms. The aryl is optionally substituted with oneor more C₁-C₂₀ alkyl, alkoxy or haloalkyl groups. Representative arylgroups include phenyl or naphthyl, or substituted phenyl or substitutednaphthyl.

“Arylalkyl” means an aryl-alkylene-group where aryl and alkylene are asdefined above. Representative arylalkyl groups include benzyl,phenylethyl, phenylpropyl, 1-naphthylmethyl, and the like, e.g., benzyl.

There is no special limitation to the di-aldehyde. The di-aldehyde maybe selected from glyoxal, malonaldehyde, succinic aldehyde andglutaraldehyde, preferably glyoxal.

There is no special limitation to the anionic monomer. The anionicmonomer can be one or more selected from a group consisting of acrylicacid, methacrylic acid, itaconic acid, maleic acid, and maleic anhydridand the salts thereof. Preferably, the anionic monomer can be acrylicacid, itaconic acid, a salt of acrylic acid, and/or a salt of itaconicacid.

In the context, there is no special limitation to the sum of the anionicmonomers, as long as a stable polymer is prepared. For example, the sumof the anionic monomers can be 0.1-50 mol %, such as 5-30 mol %, of thecopolymer, depending on the practical application, but without beinglimited to those.

In the context, there is no special limitation to the ratio ofdialdehyde, especially glyoxal, to acrylamide (G/A ratio) in thedialdehyde modified polyacrylamide. The G/A ratio can be 0.01:1-1:1(molar ratio), e.g., 0.1:1-0.8:1 (molar ratio).

In the context, the abbreviation “GPAM” used herein refers to adialdehyde modified polyacrylamide, especially glyoxal-modifiedpolyacrylamide.

There is no special limitation to the weight average molecular weight ofthe dialdehyde modified polyacrylamide, as long as it can be used as astrength agent (such as a dry strength agent). The weight averagemolecular weight of the dialdehyde modified polyacrylamide can be100,000-10,000,000 Dalton, or 500,000-2,000,000 Dalton, or800,000-1,500,000 Dalton, or 1,000,000-1,200,000 Dalton.

The dialdehyde-modified polyacrylamide can be prepared according to theknown technology, for example, the process disclosed in U.S. Pat. No.7,641,766 B2, the contents disclosed in this document being incorporatedby reference into the present application in their entirety. It shouldbe noted that, in the process of producing the dialdehyde-modifiedpolyacrylamide, a cross-linking agent and/or a chain transfer agent canbe used to provide a branched/cross-linked structure of the copolymer.

2. PAE Resin

PAE resin is generally formed by reacting a carboxylic acid, especiallya dicarboxylic acid, with a polyalkylene polyamine under conditionswhich produce a water-soluble, long-chain aminopolyamide containing therecurring groups:

—NH(C_(n)H_(2n)HN)_(x)—CORCO—

wherein n and x are more than 2 and R is the divalent, organic radicalof the dicarboxylic acid,and then reacting the polyamide with epichlorohydrin.

Dicarboxylic acids used in preparing the aminopolyamide could besaturated aliphatic dicarboxylic acids, preferably containing about 3 to8 carbon atoms, such as malonic, succinic, glutaric, adipic, and so on,together with diglycolic acid. Of these, the saturated aliphaticdicarboxylic acids having about 4 to 6 carbon atoms in the molecule,such as adipic acid, are preferred. Blends of two or more dicarboxylicacids may be used, as well as blends which include a suitable amount ofhigher saturated aliphatic dicarboxylic acids, such as azelaic andsebatic acids, as long as the resulting long-chain polyamide is watersoluble or at least water dispersible.

The polyalkylene polyamines useful in preparing the aminopolyamideinclude polyamines containing two primary amine groups and at least onesecondary amine group in which the nitrogen atoms of the secondary aminegroup are linked together by groups of the formula —C_(n)H_(2n)— (wheren is an integer of 1 to 6, and preferably 2 to 4), and the number ofsuch groups in the molecule ranges from up to eight, preferably four.The nitrogen atoms of the secondary amine group may be attached toadjacent carbon atoms in the —C_(n)H_(2n)— group or to carbon atomsfurther apart, but not to the same carbon atom. Examples of suchpolyalkylene polyamines include but are not limited todiethylenetriamine, triethylenetetramine, tetraethylenepentamine,dipropylenetriamine, and the like. These polyalkylene polyamines can beused alone or in mixtures of two or more of them.

The method for preparing the PAE resin entails reacting the obtainedaminopolyamide with epichlorohydrin in a mole ratio of epichlorohydrinto free amino groups of about 0.5:1.8, preferably 0.5:1.5 and morepreferably 1:1.25 in aqueous solution. The temperature may vary fromabout 45° C. to about 100° C.

Illustratively, the preparation of a PAE resin starting from preferredadipic acid and diethylenetriamine as well as epichlorohydrin isperformed according to the following reaction scheme:

A person skilled in the art can prepare the PAE resin of the presentinvention by referring to the above contents and the contents disclosedin, for example, U.S. Pat. No. 5,783,041. As to more detailedinformation about the preparation of PAE resin, please refer to U.S.Pat. No. 5,783,041, the contents of which are incorporated by referenceinto the present application in their entirety.

PAE resin carries a strong positive charge, tends to be retained on thefiber surface and can also further attract negatively charged GPAM,which enables PAE resin to play an excellent bridging role between fiberand anionic GPAM. As described above, the mass ratio of PAE resin toanionic dialdehyde-modified polyacrylamide is between about 5:1 andabout 1:1.6, advantageously between about 3.5:1 and about 1:1.6,preferably between about 2:1 and 1:1.23, more preferably between about1.2:1 and about 1:1. When PAE resin and anionic dialdehyde-modifiedpolyacrylamide are used in one of the foregoing ratios, it is possibleto the achieve dry tensile strength and wet tensile strength superior tothose achieved by analog products comprising cationic or amphotericdialdehyde-modified polyacrylamide.

3. Other Components

Optionally, in addition to the specific combination of PAE resin andanionic dialdehyde-modified polyacrylamide, the paper-making aidcomposition according to the invention may contain or may not containother chemical aids for paper-making, especially synthetic polymer aidsfor paper-making, e.g., polyvinyl alcohol (PVA), urea-formaldehyderesin, melamine formaldehyde resin, polyethyleneimine (PEI),polyethylene oxide (PEO), etc. The paper-making aid compositionaccording to the invention may contain or may not contain other drystrength agents. In the case that the paper-making aid compositioncontains other chemical aids for paper-making, those skilled in the artcan select the suitable kinds and amounts of the other chemical aids forpaper-making as required. The amount of the other chemical aids forpaper-making is in the range of 0˜50%, preferably 0˜20%, and morepreferably 0˜5%.

Further, as one embodiment, the paper-making aid composition may onlyconsist of a combination of the PAE resin and anionicdialdehyde-modified polyacrylamide and water as a medium.

In addition, in another embodiment, the paper-making aid composition maycontain a cationic polyacrylamide polymer as a retention aid. Thecationic polyacrylamide polymer is a copolymer formed by one or moreacrylamide monomers and one or more cationic monomers. Herein, there isno special limitation to the cationic monomers. They can be one or moreselected from the group consisting of diallyldimethylammonium chloride,N-(3-dimethylaminopropyl) methacrylamide, N-(3-dimethylaminopropyl)acrylamide, methacryloyloxyethyl trimethylammonium chloride,acryloyloxyethyl trimethylammonium chloride, methacryloyloxyethyldimethylbenzylammonium chloride, acryloyloxyethyl dimethylbenzylammoniumchloride, (3-acrylamidopropyl)trimethylammonium chloride,methacrylamidopropyl trimethylammonium chloride,3-acrylamido-3-methylbutyl trimethylammonium chloride, 2-vinyl pyridine,2-(dimethylamino)ethyl methacrylate, 2-(dimethylamino)ethyl acrylate,preferably one or more selected from the group consisting ofdiallyldimethylammonium chloride (DADMAC), N-(3-dimethylaminopropyl)methacrylamide, acryloyloxyethyl trimethylammonium chloride,2-(dimethylamino)ethyl methacrylate.

However, in one preferred embodiment, the paper-making aid compositionof the present invention may not contain a cationic polyacrylamidepolymer, as the inventors have surprisingly found that the paper-makingaid composition not comprising a cationic polyacrylamide polymeraccording to the present invention might lead to a better tensilestrength.

As described above, another aspect of the present invention is toprovide a paper-making process, comprising the steps of:

-   -   (a) providing a pulp slurry; simultaneously or before or after    -   (b) providing the above paper-making aid composition;    -   (c) adding the paper-making aid composition into the paper        slurry to obtain a paper stock;    -   (d) forming the paper stock obtained in the step (c) to obtain a        wet paper web;    -   (e) pressing and draining the wet paper web obtained in the        step (d) to obtain a wet paper sheet; and    -   (f) drying the wet paper sheet obtained in the step (e) to        obtain a paper sheet.

In the context, “paper-making process” or “process for paper-making”means a method of making paper products from pulp comprising forming anaqueous cellulosic papermaking furnish, draining the furnish to form asheet and drying the sheet.

In the context, “pulp slurry” or “pulp” is intended to mean a productobtained from a pulping process. “Pulping” involves a production processof dissociating the plant fiber raw materials by a chemical method or amechanical method, or a combination of the both, to form a paper pulpwith an inherent color (unbleached pulp) or further to form a bleachedpulp. The pulp can be any known pulp, including but not limited to,mechanical pulp, chemical pulp, chemical mechanical pulp, and recycledwaste paper pulp, for example, a pulp containing mechanical pulp and/orrecycled fiber.

In the context, the pulp is subject to the pulping and additiveadjustment, producing a fiber suspension which can be used inhandsheeting. Such a fiber suspension is called as “paper stock”, so asto be distinguished from the paper slurry which is not subject to apulping and an additive adjustment.

In the context, “wet paper sheet” refers to a product obtained after thepulp stock passed the headbox, the forming section and the press sectionto be formed and partially drained, wherein the dryness of the wet papersheet can be in a range of from about 35% to 50%. For the sake ofclarity, the product which comes from the forming section but is notsubject to drainage in the press section is called as “wet paper web”,which can have a dryness in a range of from about 15% to 25%.

In the context, “paper sheet” refers to a product obtained after the wetpaper sheet is dried in the dryer section. The dryness of the papersheet can be in a range of from about 92% to 97%.

In general, the paper-making process according to the invention can becarried out by the following procedure, but not limited to this, i.e.,the paper-making process according to the invention can be also carriedout by other known paper-making procedures in the art.

1. The treatment before the paper stock flowing onto the wire comprises:

(1) the preparation of paper stock: the paper slurry can be made into apaper stock, and the preparation of the paper stock comprises pulpingand additive adjustment (adding additives such as sizings, fillers,pigments and aids). The paper slurry is first subject to pulping whereinthe fiber of the paper slurry undergoes treatments such as necessarycutting, swelling and fine fibrosis, so as to render the paper havingphysical properties and mechanical properties required by a certain sortof paper and meeting the requirements of a paper-making machine. Inorder to render the paper sheet useful for writing and resistant toliquid impregnation, improve the paper color, white and tone, increasethe opaqueness of the paper and increase the printing performance of thepaper, etc., the paper slurry can undergo sizing, adding filler andstaining. Furthermore, various chemical aids can be added to provide thepaper with some special properties (for example, enhancing the drystrength, wet strength and eliminating bubbles).

(2) supplying the paper stock to the slurry supply system: the paperstock is supplied into the slurry supply system, undergoes treatmentssuch as storing, screening, purifying, de-slagging, de-sanding,de-gasing, and dischages the metal, nonmetal impurities, fiber bundle,lump and air, etc., so as to avoid the adverse effect on the quality ofthe paper and hinder the paper-making process. The slurry pass undergoesslurry proportion, dilution, concentration adjustment, dosage andpressure elimination, and then flow into the head box and onto the wirefor making paper.

2. The paper-making of paper comprises:

(1) stock flow approching: the paper stock is delivered to the formingsection (wire section) through the headbox. The headbox is useful indispering the fiber homogeneouly and flowing the slurry onto the wiresmoothly. The additives for paper making, such as the dry strength aidsfor paper, the wet strength aids for paper, can be added in the processof stock flow approching.

(2) forming: in the forming section, the paper stock delivered byheadboxis formed into a wet paper web by draining on the wire. Theforming section is also referred to as wire section. The dryness of thewet paper web can be in range of from about 15% to 25%.

(3) pressing and draining: in the pressing section, the wet paper webfrom the forming section is subject to a mechanical pressing to form awet paper sheet. The dryness of the wet paper sheet can be in a range offrom about 35% to 50%.

The steps (d) and (e) can be carried out by the above 2.(2) and 2.(3).

(4) drying: in the dryer section, the wet paper sheet from the pressingsection is dried with a dry cylinder to form a paper sheet. The drynessof the paper sheet can be in a range of from about 92% to 97%.

The step (f) can be carried out by the above 2.(4).

Moreover, the paper sheet can undergo, as required, finishing proceduressuch as calendering, winding and cutting, paper-sorting or rewinding,packaging, etc., so as to produce the paper sheet in to a finished paperin the form of flat or roller. Additionally, in order to improve thequality of the paper sheet, surface sizing, coating and online softcalender or offline supercalender can be carried out in the dryersection.

In the paper-making process, the paper slurry provied by a paper stockpreparation system is generally subject to a slurry supply system(undergoing a treatment before the paper stock flows onto the wire), theheadbox and the forming section, the press section, dryer section, etc.

The paper-making aid composition is added into the pulp slurry in anamount of between about 0.01 kg/ton dry fiber and 50 kg/ton dry fiber,e.g., 0.1 kg/ton dry fiber and 10 kg/ton dry fiber, based on the weightratio of the sum of the dialdehyde-modified polyacrylamide and PAE resinto the dry fiber in the pulp slurry.

EXAMPLES

The invention is described in more detail by referring to the followingExamples and Comparative Examples, but is not limited to these Examples.

1. Paper-Making Process and Characterization of Paper (a) Method forMaking Hand Sheet

The pulp slurry (thick stock) is obtained from a paper mill. The thickstock comprises a mixed slurry of softwood bleached kraft pulp andhardwood bleached kraft pulp, or other pulp, as main component.Sheet-making is performed after the thick stock is diluted with tapwater or white water from paper-making plant to a concentration of about0.7%.

Semi-automatic Tappi standard sheet-making machine, provided byFRANK-PTI Co., is used as the sheet-making machine. The specific testmethod is described in T205 Introduction sp-02. To the diluted pulp, afixing agent, test additives and retention aids are added successivelyat a rotation speed of about 800 rpm.

The pulp added with the agents is poured into a forming cylinder ofpaper-making machine and undergoes filtering and forming. Afterwards,the forming cylinder is opened, and a bibulous paper is taken to coverthe wet paper sheet which is then covered with a flat clamp to removepart of water. Then the paper sample is transferred to a new bibulouspaper which is then covered with stainless steel clamp, onto which abibulous paper is covered again, the wet paper sample is thusaccumulated. When accumulating 5 to 10 paper samples, they are providedinto a special press machine to perform a two-section pressing, furtherremoving water from paper.

The pressed paper is transferred to a constant temperature and humiditylab (about 50% humidity at 23° C.), and every single paper sample isplaced into a special metal ring. Piling up the metal rings and placinga heavy object onto the metal ring where the paper sample lies on. Afterair drying for about 24 hours, the paper sample can be peeledsuccessively from stainless steel clamp for corresponding test.

(b) Test Method for Internal Bonding Strength

The principle of the internal bond impact tester is to measure theenergy required to separate the paper sheet by a mechanical equipment soas to reflect the magnitude of the internal bonding strength. Themeasurement of the internal bonding strength is to express the resistantforce that is required to overcome for separating the single or multiplefiber layer(s), which is frequently used to discuss the delaminationproblem of the paper sheet or paperboard. The test method adopted in theexperiment comprises the determination of the force applied by apendulum to splitting the paper along Z-direction. When the fibers of ahand sheet align in X-Y plane, the exhausted energy is mainly used forthe bonding of the fiber, and the length of the fiber and the strengthof the fiber itself have no influence on the Scott bonding.

The equipment used in the experiment was purchased from PTI company. Thetest method refers to Tappi T569.

For a test, a paper with a size of around 25.4 mm×200 mm is cut outpreviously, and then tape and paper sample are attached to a basefollowing a sequence of tape-paper sample-tap, and the double-sidedadhesive tape and the paper sample are attached to each other closely byapplying a force. Afterwards, a pendulum is released to knock andseparate the paper sample when the equipment automatically records theforce that is required to separate the bonding of the fiber layers foreach time, expressed in kg·cm/in², J/m².

(c) Determination of Viscosity

Brookfield Programmable LVDV-II+viscometer, manufactured by BrookfieldEngineering Laboratories, Inc, Middleboro, Mass., is utilized in thisexperiment.

0-100 cps, measured by Spindle 1 at 60 rpm100-1000 cps, measured by Spindle 2 at 30 rpm1000-10000 cps, measured by Spindle 3 at 12 rpm

2. Preparation Example

(a) Preparation of PAE resin

PAE resin used in the examples and comparative examples was polyamidepolyamine epichlorohydrin, manufactured and sold by Nalco. Co., whichwas prepared according to the following process:

About 82 kg of diethylenetriamine, about 15 kg of distilled water andabout 1 kg of p-toluenesulfonic acid were put into a reaction vessel.Then, about 110 kg of adipic acid was added portion-wise with stirring,and the mixed solution was allowed to automatically warm up to about125° C. After fractionation of water, the system was further heated toabout 150˜160° C., and kept the temperature for about 3 h. When thetotal amount of distillated water and amine was approximately 35 kg, thereaction tended to complete. Then, the system was cooled to below 100°C., and about 160 kg of water was added and stirred until a uniformbright red, transparent viscous liquid was obtained, having a solidscontent of about 50% and a viscosity (25° C.) of about 600˜1000 mPa·s.About 400 kg of water was added to the above-obtained polyamide, andabout 80 kg of epichlorohydrin was added with stirring. After reactingat 70° C. for about 1˜2 h until the required viscosity was achieved,acetic acid was added to adjust pH to about 3˜5, giving the PAE resin.

The basic properties of the PAE resin:

Active substance: polyamide polyamine epichlorohydrinSolids content: 25%Viscosity: 600˜1000 mPa·spH value: 3˜5

(b) Preparation of Glyoxalated Polyacrylamide (GPAM Copolymer) Solution

GPAM copolymer used in the examples and comparative examples wasprepared according to the following process:

(1) Synthesis of Polyacrylamide Base Polymer 1 (Intermediate 1)

To a 2 L three-neck flask with a heating and a cooling tube, about 90 gdeionized water, about 0.1 g ethylenediamine tetraacetic acid (EDTA) andabout 160 g diallyldimethylammonium chloride (DADMAC) were added. Aninitiator comprising about 4 g ammonium persulfate and about 16 gdeionized water was added once the obtained solution was heated to about100° C. and the addition took about 137 minutes to complete. Theaddition of monomer phase containing about 625 g acrylamide(concentration 50%) was started after adding the initiator for about 2minutes. The addition of monomer phase took about 120 minutes tocomplete. After completing the addition of the initiator, the solutionwas incubated at about 100° C. The reaction ended in about 1 hour,affording an intermediate 1 with a solids content of about 41 wt % and aviscosity of about 2000 cps, wherein the concentration of cationicmonomeric units was about 12 mol %.

(2) Synthesis of Polyacrylamide Base Polymer 2 (Intermediate 2)

To a 2 L three-neck flask with a heating and a cooling tube, about113.486 g deionized water, about 16.25 g 48% sodium hydroxide aqueoussolution, about 26.27 g 75% phosphoric acid solution, about 7.6 g sodiumformate, and about 0.1 g ethylenediamine tetraacetic acid were added. Aninitiator comprising about 4.4 g ammonium persulfate and about 13.2 gdeionized water was added dropwise once the obtained solution was heatedto about 100° C. and the addition took about 130 minutes to complete.The addition of a mixed solution containing about 768.401 g 50%acrylamide and about 20.6 g 100% acrylic acid was started after addingthe initiator for about 2 minutes. The addition took about 120 minutesto complete. After completing the addition of the initiator, thesolution was incubated at about 100° C. The reaction ended in about 2hours, affording an intermediate 2 with a solids content of about 41 wt%, a viscosity of about about 1380 cps, and a molecular weight of about15,000˜25,000, wherein the concentration of anionic monomeric units wasabout 5 mol %.

(3) Synthesis of Polyacrylamide Base Polymer 3 (Intermediate 3)

To a 2 L three-neck flask with a heating and a cooling tube, about200.78 g deionized water, about 16.25 g 48% sodium hydroxide aqueoussolution, about 26.27 g 75% phosphoric acid solution, about 7.6 g sodiumformate, about 0.1 g ethylenediamine tetraacetic acid and about 109.4 gdiallyldimethylammonium chloride (concentration 62%) were added. Aninitiator comprising about 4.4 g ammonium persulfate and about 13.2 gdeionized water was added dropwise once the obtained solution was heatedto about 100° C. and the addition took about 130 minutes to complete.The addition of a mixed solution containing about 609.5 g 50% acrylamideand about 12.5 g 100% acrylic acid was started after adding theinitiator for about 2 minutes. The addition took about 120 minutes tocomplete. After completing the addition of the initiator, the solutionwas incubated at about 100° C. The reaction ended in about 2 hours,affording an intermediate 3 with a solids content of about 39 wt %, aviscosity of about about 530 cps, and a molecular weight of about15,000˜20,000, wherein the concentrations of cationic and anionicmonomeric units were respectively about 8.5 and 3.5 mol %.

(4) Synthesis of Glyoxalated Cationic Polyacrylamide Copolymer 1 (GPAM1)

To a 2 L glass container, about 727 g deionized water, about 195 g theabove intermediate 1 and about 49 g 40% glyoxal solution were separatelyadded and mixed at about 25° C. in a mechanical stirrer for about 15minutes. The pH value of the obtained solution was adjusted to about 8.4with a 48% sodium hydroxide solution. During the reaction, samples weretaken for the determination of the viscosity until a product with aviscosity of about 18 cps was obtained. The obtained product wasadjusted with a 50% sulfuric acid until pH value is about 3, affording amodified polymer having a solids content of about 10 wt % and amolecular weight of about 1,200,000 Dalton. The final product was markedwith “GPAM 1”.

(5) Synthesis of Glyoxalated Anionic Polyacrylamide Copolymer 2 (GPAM 2)

To a 2 L glass container, about 783.5 g deionized water and about 155.5g the above intermediate 2 were added, and the obtained solution wasadjusted to have a pH value of about 9 with about 3 g 48% sodiumhydroxide solution. Then, about 47.2 g 40% glyoxol solution was added,and the pH value was adjusted to about 8.5 with about 6.8 g 5% sodiumhydroxide solution. The reaction was carried out at a normaltemperature, and a viscometer was used to monitor the viscosity of thereaction solution. At the beginning, the viscosity of the reactant wasabout 4˜5 cps. When the reactant reached a viscosity of about 14 cps,50% sulfuric acid was added to adjust the pH value of the product to beabout 3, so as to obtain a polymer having a solids content of about 8 wt% and a molecular weight of about 1,200,000 Dalton. The final productwas marked with “GPAM 2”.

(6) Synthesis of Glyoxalated Amphoteric Polyacrylamide Copolymer 3 (GPAM3)

To a 2 L glass container, about 732.63 g deionized water and about 205.5g of the above intermediate 3 were added, and the obtained solution wasadjusted to have a pH value of about 9 with about 4.07 g 48% sodiumhydroxide solution. Then, about 50.3 g 40% glyoxol solution was added,and the pH value was adjusted to about 8.5 with about 7.5 g 5% sodiumhydroxide solution. The reaction was carried out at a normaltemperature, and a viscometer was used to monitor the viscosity of thereaction solution. When the reactant reached a viscosity of about 18cps, 50% sulfuric acid was added to adjust the pH value of the productto be about 3, so as to obtain a polymer having a solids content ofabout 10 wt % and a molecular weight of about 1,000,000 Dalton. Thefinal product was marked with “GPAM 3”.

(c) Preparation of Cationic Polyacrylamide Copolymer

The cationic polyacrylamide copolymer used in the examples was preparedaccording to the following process:

To a 2 L three-neck flask equipped with a heating and a cooling tube,about 21.1 g deionized water, about 546 g acrylamide (concentration50%), about 10 g oxalic acid, about 15 g urea, about 105 gacryloyloxyethyl trimethylammonium chloride (DMAEA⋅MCQ), about 20 gcrude oil and about 15 g of sorbitan monooleate were added. The solutionwas heated to about 45° C. and rapidly stirred until fully dissolved.Subsequently, nitrogen gas was charged, and about 0.3 gazobisisobutyronitrile was added. The reaction was carried out at about45° C. for about 3 hours until complete, to obtain a cationicpolyacrylamide copolymer having a solids content of about 35 wt % and aviscosity of about 1500 cps.

3. Examples Example 1

The PAE resin solution and GPAM 2 were respectively diluted 15 times byadding the ionized water. The diluted PAE resin solution and GPAM2solution were added into the furnish in sequence in an activeconcentration mass ratio of about 1.25:1. The interval of adding thecomponents was about 60 s. The hand sheet samples of the invention wereprepared according to the hand sheet preparation method as describedabove with two different dosages (about 3 kg/ton or about 6 kg/ton). Thethick stock used in this Example was a mixed slurry of softwood bleachedkraft pulp and hardwood bleached kraft pulp.

It should be noted that the dosage of the tested additive herein refersto the amount of the active ingredient in the solution (agent) relativeto the dry fiber in the pulp slurry. The meaning of dosage also appliesto the following examples. The composition and amount ratios ofdifferent paper-making aids used in the example and the measuredproperties were listed in Table 1.

TABLE 1 Composition and ratios of different paper-making aids andmeasured properties Dosage of Dry Dry Wet Wet Composition and paper-tensile tensile tensile tensile ratio (mass ratio) of making strengthstrength strength strength Cohesion Cohesion paper-making aid aids, kg/tN · m/g increase % N · m/g increase % J/m² increase % Blank 21.1 1.4474.48 100% PAE 3 23.77 12.7 4.94 243.1 89.68 20.4 100% PAE 6 26.11 23.76.15 327.1 97.28 30.6 100% GPAM2 3 20.02 −5.1 1.66 15.3 71.44 −4.1 100%GPAM2 6 20.6 −2.4 1.69 17.4 69.92 −6.1 PAE:GPAM2_1.25:1 3 26.34 24.85.25 264.6 103.36 38.8 PAE:GPAM2_1.25:1 6 32.17 52.5 7.49 420.1 133.7679.6

As seen from Table 1, the use of a combination of PAE resin and anionicGPAM2 in a mass ratio of about 1.25:1 according to the present inventionas strength agent will result in better dry tensile strength and wettensile strength and higher tensile strength increase as compared withthe use of the PAE resin or GPAM2 alone at the same dosage. Meanwhile,in the case of comparable dry tensile strength and wet tensile strengthand tensile strength increase, the amount of the strength aids,especially the amount of polluting PAE resin, can be significantlyreduced by using the paper-making aid according to the present inventionas strength agent.

Example 2

The PAE solution and GPAM 2 were respectively diluted 15 times by addingthe ionized water, and added into the furnish in sequence in differentactive concentration mass ratios (see Table 2 below). The interval ofadding the components was about 60 s. The hand sheet samples of theinvention were prepared according to the hand sheet preparation methodas described above with two different dosages (about 2 kg/ton or about 4kg/ton). The thick stock used in the Example was a mixed slurry ofsoftwood bleached kraft pulp and hardwood bleached kraft pulp.

TABLE 2 Optimization of the combination of PAE resin and GPAM2 Wet DryPaper-making Dosage Wet tensile Dry tensile aid of the tensile strengthtensile strength composition aid strength, increase, strength, increase,(PAE:GPAM2) kg/t N · m/g % N · m/g % Blank 0 1.79 33.17 1:0 2 4.75 165.434.9 5.2 1:0 4 6.34 254.2 36.94 11.4 0:1 2 1.94 8.4 33.08 −0.3 0:1 42.23 24.6 34.35 3.6 3.5:1  2 4.89 173.2 36.04 8.7 3.5:1  4 6.87 283.839.78 19.9 2:1 2 4.61 157.5 35.57 7.2 2:1 4 7.2 302.2 40.2 21.2 1:1 24.78 167.0 36.3 9.4 1:1 4 7.27 306.1 42.98 29.6 1:2 2 3.74 108.9 35.436.8 1:2 4 4.8 168.2 36.74 10.8 1:3 2 2.93 63.7 35.29 6.4 1:3 4 3.8 112.336.14 9.0

As seen from Table 2, when the mass ratio of PAE resin and anionic GPAM2is in the range as claimed in the present invention, the use of acombination of these two components as strength agent will result inbetter tensile strength as compared with the use of the PAE resin orGPAM2 alone. However, when the mass ratio is below or equal to about 1:2(for example, 1:2 or 1:3), as the mass of GPAM2 increases, the wettensile strength increase will significantly decrease, even inferior tothe use of the PAE resin.

Example 3

This example was carried out on the basis of Example 2 making a furthercomparison in the vicinity of the active mass ratio of PAEresin:GPAM2=1:1 so as to obtain the optimum mass ratio. In this example,the operation of Example 2 was repeated except for using furtherspecified mass ratios as shown in Table 3 below. The data of tensilestrength as measured were listed in this table.

TABLE 3 Further optimization of active mass ratio of PAE resin to GPAM2Wet Dry Paper-making Dosage Wet tensile Dry tensile aid of the tensilestrength tensile strength composition aid strength, increase, strength,increase, (PAE:GPAM2) kg/t N · m/g % N · m/g % Blank 0 1.97 33.2 9.7:1  4 7.05 257.9 37.86 14.0 5:1 2 5.02 154.8 35.57 7.1 5:1 4 7.48 279.739.22 18.1 1.08:1   2 4.98 152.8 37.13 11.8 1.08:1   4 7.92 302.0 41.825.9   1:1.23 2 4.46 126.4 36.11 8.8   1:1.23 4 7.23 267.0 41.12 23.9

As summarized from Table 1, Table 2 and Table 3, it can be seen that,when the active mass ratio of the PAE resing to GPAM2 is controlledbetween about 1.2:1 and 1:1, the optimum dry and wet tensile strengthcan be achieved.

Example 4

This example was carried out to compare the addition manners of the PAEresing and GPAM2. In this example, the PAE resin and GPAM2 as preparedabove were added to a pulp in a active mass ratio of about 1:1 in twodifferent manners, i.e. separate addition (separately) or pre-mixingaddition (pre-mixing). When the PAE resin and GPAM2 were addedseparately, PAE resing was added first and then, after about 60 s, GPAM2was added.

TABLE 4 Comparison of addition manners of PAE resin and GPAM2 Wet DryPaper-making Dosage tensile Wet tensile tensile Dry tensile aidcomposition Addition of aid strength, strength strength, strength (massratio) manners kg/t N · m/g increase, % N · m/g increase, % Blank Blank0 1.67 30.59 Only PAE \ 2 4.1 145.5 33.9 10.0 4 6.1 265.3 34.6 12.1PAE:GPAM2 ≈ 1:1 separately 2 4.91 194.0 36.4 17.5 separately 4 7.95376.0 40.52 29.9 pre-mixing 2 3.71 122.2 34.5 11.8 pre-mixing 4 4.48168.3 36.1 16.6

As seen from Table 4, separately adding the PAE resin and GPAM2performed obviously better than the pre-mixing manner.

Example 5

Comparison was made between different charged GPAM when combining with64897 as strength solutions.

PAE resing, GPAM 1, GPAM 2 and GPAM 3 were diluted 15 times respectivelyusing ionized water first, the PAE resin combined with GPAM1, GPAM2 andGPAM3 were used as test additives with about 1.2:1 mass ratio in twodosages (about 3 kg/t and about 6 kg/t) in the preparation of thehandsheet samples of the invention according to the handsheetpreparation method described above. The other steps of the experimentare the same as Example 1.

As seen from Table 5, the combination of PAE resin with anionic GPAM 2performed better than the combination of PAE resin with cationic oramphoteric GPAM copolymer.

TABLE 5 Dosage Wet Dry Dry Paper-making of the tensile Wet tensiletensile tensile aids composition aid strength, strength strength,strength Cohesion Cohesion (mass ratio) kg/t N · m/g increase, % N · m/gincrease, % J/m² increase % Blank 0 1.51 21.43 71.44 PAE:GPAM1 3 4.85221.2 24.15 12.7 89.68 25.5 (1.2:1) PAE:GPAM1 6 6.22 311.9 30.38 41.8114 59.6 (1.2:1) PAE:GPAM2 3 5.06 235.1 26.5 23.7 97.28 36.2 (1.2:1)PAE:GPAM2 6 7.34 386.1 31.55 47.2 121.5 70.1 (1.2:1) PAE:GPAM3 3 4.86221.9 25.6 19.5 95.76 34.0 (1.2:1) PAE:GPAM3 6 6.42 325.2 29.46 37.5115.5 61.7 (1.2:1)

Example 6

This example was focused on a comparison between a dual componentstrength solution composed of the PAE resin and GPAM 2 and a ternarycomponent strength program composed of PAE resin, GPAM 2 and cationicpolyacrylamide copolymer.

The PAE resin and GPAM2 was added with about 1:1 mass ratio, followed bythe cationic polyacrylamide copolymer. The addition of chemistries wasdone in about 60 second intervals. The dual and ternary componentstrength solutions were used as test additives in the preparation of thehandsheet samples of the invention according to the handsheetpreparation method described above. Specific dosage of the additives waspresented in Table 6. The thick stock used in the Example was a mixedslurry of softwood bleached kraft pulp and hardwood bleached kraft pulp.

As seen from Table 6, the ternary component strength program composed ofthe PAE resin, GPAM2 and the cationic polyacrylamide copolymer was lesseffective than the dual component strength program composed of the PAEresin and GPAM2. Moreover, the higher dose of the cationicpolyacrylamide copolymer was the worst with respect to strengthperformance.

TABLE 6 Dosage Dosage of Wet Dry Paper-making of the cationic tensileWet tensile tensile Dry tensile aid composition aid polyacrylamidestrength, strength strength, strength (mass ratio) kg/t copolymer, kg/tN · m/g increase, % N · m/g increase, % PAE:GPAM2 0 0 1.97 33.2 (1:1) 20 8.17 314.7 43.94 32.3 2 0.05 8.02 307.1 42.95 29.4 2 0.1 7.56 283.841.8 25.9 2 0.2 7.33 272.1 41.89 26.2

Example 7

Application of the invented strength solution in carton board furnishwas studied in this experiment, wherein the active mass ratio of PAEresing to GPAM 2 was about 1:1, the addition of chemistries was done inabout 60 second intervals. The new strength solutions were used as testadditives in two dosages (about 3 kg/t and about 6 kg/t) in thepreparation of the handsheet samples of the invention according to thehandsheet preparation method described. The thick stock used in theExample was a mixed slurry of bleached chemi-mechanical pulp, de-inkedpulp, hardwood bleached kraft pulp and waste paper pulp.

As seen from Table 7, the new strength solution composed of the PAEresin and GPAM 2 showed comparable wet strength but higher dry strengththan using the PAE resin alone.

TABLE 7 Wet Dry Paper-making Wet tensile Dry tensile aid Dosage tensilestrength tensile strength composition of the aid strength, increase,strength, increase (mass ratio) kg/t N · m/g % N · m/g % Blank 0 0.6616.91 PAE:GPAM2 3 4 506 21.84 29 (1:1) PAE:GPAM2 6 5.87 789 25.52 51(1:1) PAE 3 4.28 548 21.19 25 P 6 5.98 806 23.12 36

1. A paper-making aid composition comprising an anionicdialdehyde-modified polyacrylamide and a polyamidepolyamine-epichlorohydrin (“PAE”) resin present in the paper-making aidcomposition at a weight ratio of PAE resin to anionicdialdehyde-modified polyacrylamide of from about 5:1 to about 1:1.6. 2.The paper-making aid composition of claim 1, wherein the PAE resin andthe anionic dialdehyde-modified polyacrylamide are present at a weightratio of PAE resin to anionic dialdehyde-modified polyacrylamide of fromabout 3.5:1 to about 1:1.6.
 3. The paper-making aid composition of claim1, wherein the paper-making aid composition has active content ofdialdehyde-modified polyacrylamide and the PAE resin of from about 10 wt% to about 50 wt %.
 4. The paper-making aid composition of claim 1,wherein, except for the anionic dialdehyde-modified polyacrylamide andthe polyamide polyamine-epichlorohydrin resin, the amount of otherchemical aids for paper-making in the paper-making aid composition isfrom about 0 wt % to about 50 wt %.
 5. The paper-making aid compositionof claim 1, wherein the paper-making aid composition is free of cationicand amphoteric dialdehyde-modified polyacrylamide.
 6. The paper-makingaid composition of claim 1, consisting of the PAE resin, the anionicdialdehyde-modified polyacrylamide, and water.
 7. The paper-making aidcomposition of claim 1, wherein the anionic dialdehyde-modifiedpolyacrylamide is glyoxalated polyacrylamide.
 8. The paper-making aidcomposition of claim 1, wherein the paper-making aid composition is freeof cationic polyacrylamide polymer.
 9. A process for increasing tensilestrength of paper comprising adding an anionic dialdehyde-modifiedpolyacrylamide and a polyamide polyamine-epichlorohydrin (“PAE”) resinat a weight ratio of PAE resin to anionic dialdehyde-modifiedpolyacrylamide of from about 5:1 to about 1:1.6 to a slurry comprisingpulp in a pulping process and/or a paper-making process.
 10. The processof claim 9, wherein the anionic dialdehyde modified polyacrylamide andthe PAE resin are added separately to the paper-making process.
 11. Theprocess of claim 9, wherein the pulp comprises softwood bleached kraftpulp and hardwood bleached kraft pulp.
 12. A paper-making processcomprising: (a) adding an anionic dialdehyde-modified polyacrylamide anda polyamide polyamine-epichlorohydrin (“PAE”) resin at a weight ratio ofPAE resin to anionic dialdehyde-modified polyacrylamide of from about5:1 to about 1:1.6 into pulp slurry to obtain a treated stock; (b)forming the treated stock obtained in step (a) to obtain a wet paperweb; (c) pressing and draining the wet paper web obtained in step (b) toobtain a wet paper sheet; and (d) drying the wet paper sheet obtained instep (c) to obtain a paper sheet.
 13. The paper-making aid compositionof claim 1, wherein, except for the anionic dialdehyde-modifiedpolyacrylamide and the polyamide polyamine-epichlorohydrin resin, theamount of other chemical aids for paper-making in the paper-making aidcomposition is from about 0 wt % to about 20 wt %.
 14. The paper-makingaid composition of claim 1, wherein, except for the anionicdialdehyde-modified polyacrylamide and the polyamidepolyamine-epichlorohydrin resin, the amount of other chemical aids forpaper-making in the paper-making aid composition is from about 0 wt % toabout 5 wt %.
 15. The paper-making aid composition of claim 1, whereinthe paper-making aid composition is free of amphotericdialdehyde-modified polyacrylamide.
 16. The paper-making aid compositionof claim 1, further comprising a retention aid.
 17. The process of claim9, wherein the slurry is thick stock.
 18. The process of claim 9,wherein the slurry is carton board furnish.
 19. The paper-making processof claim 12, wherein the pulp slurry is thick stock.
 20. Thepaper-making process of claim 12, wherein the pulp slurry is cartonboard furnish.